Specific Aims Transforming growth factor (TGF-) superfamily signaling in endothelial cells regulates essential components of angiogenesis and vascular morphogenesis, including proliferation and capillary tube formation. TGF- superfamily ligands exert their regulatory effects through the endothelial cell specific TGF- receptor complex, ALK1 (type I receptor) and endoglin (co-receptor), along with the ubiquitous type I TGF- receptor, ALK5, to activate the canonical Smad 1/5/8 and Smad 2/3 pathways, respectively. TGF- ligands also signal through non-Smad pathways such as MAPKs and PI3K/Akt, although the underlying mechanisms remain obscure. A critical role for endoglin and ALK1 in TGF- signaling in endothelial cells is supported by their mutation resulting in the human vascular disease, hereditary hemorrhagic telangiectasia (HHT1 and 2), embryonic lethal phenotype due to defects in angiogenesis when either endoglin or ALK1 is targeted for deletion in mice, and by the elevated expression of endoglin during inflammation and tumor-induced angiogenesis. While important biological roles for endoglin have been established, the molecular basis for endoglin function in vascular biology remains poorly characterized. Here in vitro angiogenesis assays were employed to investigate precisely when and how endoglin regulates endothelial capillary sprouting and tube formation. Comparison of endoglin-null and wild type endothelial cells revealed that endoglin differentially regulates the stability of capillary sprouts and tubes in response to its physiologically relevant high-affinity ligands, TGF- and BMP-9. Specifically, TGF- resulted in regression of the capillary sprouts and tube structures, primarily through suppression of endoglin-dependent Akt signaling. Conversely, endoglin enhanced Akt signaling in response to BMP-9 to promote capillary stability. These outcomes are attributed to the association between endoglin and the scaffolding/trafficking protein, GIPC, since disrupting their interaction abrogated such endoglin-dependent effects. Given that I recently reported the enhancement of Smad 1/5/8 signaling through GIPC and endoglin, there exists a potential crosstalk between Akt and endoglin-dependent Smad 1/5/8 signaling, which I propose to investigate. Lastly, I discovered a novel interaction between endogenous endoglin and Akt, a finding that will likely yield new facets of endoglin biology. Based upon these preliminary data, I propose the following hypothesis: Endoglin associates with GIPC to promote angiogenesis by stabilizing endothelial capillaries via BMP-9-dependent Akt activation while destabilizing capillaries via TGF--dependent suppression of Akt signaling and cell survival mechanisms. This hypothesis will be addressed by the objectives outlined in two specific aims.